This invention relates to a toothed power transmission belt and pulley drive.
U.S. Pat. No. 2,507,852, to R. Y. Case, describes a power transmission belt comprising an inextensible tensile member having teeth bonded to one side and a protective jacket fabric covering the teeth. The teeth are preferably made of an elastomeric material, such as rubber, and the belt may also include a backing layer of identical or similar material to that which the teeth are constructed.
Many different elastomeric materials have been utilized for the construction of belts made in accordance with the Case patent, some of the more common materials being neoprene and polyurethane. These belts are designed to mesh with toothed pulleys that are constructed of a material having a higher Young's modulus than the elastomeric material used for the construction of the belt. The conventional toothed belt, as described in the Case patent, utilizes a tooth cross-sectional configuration that is essentially trapezoidal and which is very similar to a conventional rack tooth. Many attempts have been made to alter the belt and pulley teeth configurations to relieve the problem of belt failure. In such trapezoidal tooth belts, the common failure is that of tooth shear due to stress concentration. In seeking to reduce tooth shear, U.S. Pat. No. 3,756,091 to H. Miller, discloses belt teeth having a cross-sectional configuration which approximates the contour of the one-half order isochromatic fringe in a belt tooth under a defined rated load. The pulley grooves in accordance with the Miller patent are in mating engagement with and are substantially conjugate to the belt teeth. The belt having substantially curvilinear teeth in accordance with the Miller patent reduced belt tooth shear and increased horsepower capacity. One mode of failure in the belt configuration according to Miller may occur because of land wear in the belt, especially with small diameter pulleys. Land wear between the belt teeth is due to abrasion of the protective layer and exposure of the tensile member by action of the pulley tooth against the belt. This land wear leads to premature failure due to a detachment of the teeth from the tensile member and/or a break in the tensile member.
U.S. Pat. No. 4,037,485, to Hoback, proposes a solution to the land wear problem. As disclosed in the Hoback patent, the dimensional relationship of the belt teeth and grooves and the pulley teeth and grooves is such that in the longitudinal extent of the belt between the pulleys, the height of the belt teeth is greater than the depth of the pulley grooves while as the belt travels around the pulleys, the extreme outwardly facing portions of the belt teeth which confront the pulleys comes into contact with the portions of the toothed pulley which define the bottom of the pulley grooves. At the same time, Hoback discloses that the belt teeth are compressed to reduce their height so that the extreme radially outwardly facing portions of the pulley teeth come into contact with the portions of the belt disposed between the belt teeth which define the bottoms of the belt grooves.
Relatively early in the operating life of the toothed power transmission belt and pulley as disclosed in Hoback, the compressive engagement of the elastomeric teeth against the bottom of the pulley groove results in a substantially permanent deformation of the belt tooth. This deformation is accelerated at elevated operating temperatures such as occur in automotive applications. The deformation results in a significant permanent decrease in the tooth height. The deformed teeth no longer support the tensile member as desired by Hoback. Subsequently the same wear pattern develops in the toothed belt and pulley drive according to Hoback as in the toothed power transmission configuration according to the Miller patent.